Use a script to serve the images.
I do not have the exact scriipt, but in general: Refer to the image by an ID value. In the image source you use the php script serving the image:
<img src=\"images.php?id=$id\">
and you let the script check wether a person is logged in. If he is, he starts an image stream after passing image header info.
Look at keywords: "header" "Images stream"